There are several interpretations of what constitutes one acid:

The Arrhenius definition: by the 1884 an interpretation of Svante Arrhenius (Sweden), an acid is a material that have the right to release a proton or hydrogen ion (H+).

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The Lewis definition: A Lewis acid is one that can accept a pair of electron and form a name: coordinates covalent bond.

The Brønsted-Lowry definition: Brønsted said that every acid-base reaction involve the deliver of an H+ ion, or proton. Water reacts with itself, for example, by carrying an H+ ion native one molecule to another to type an H3O+ ion and also an OH- ion. According to this theory, an acid is a "proton donor" and a basic is a "proton acceptor."

For currently we will stick with the Brønsted-Lowry definition. And we will now tie the concept of acids and also bases right into equilibrium:

Let"s start by looking at the Ionization the Water:

2H2O(l) H3O+(aq) + OH-(aq)

This reaction go fit the Brønsted-Lowry meaning since one water molecule is acting together an acid "donating" a proton and the other is acting as a basic "accepting" the proton:


If we now write this reaction into an equilibrium expression:


But remember one of our rules about the creating of equilibrium expressions states that we carry out not incorporate solvents (liquids) together their concentration stays pretty constant during a reaction for this reason the water part of the equation is basically equal to 1 and also thus "disappears".

The resulting equation is given a one-of-a-kind designation KW called the ionization constant of water:


The worth of this continuous at 25oC is 1.0 x 10-14. As you deserve to see native the very little value of KW, water is not very dissociated in ~ room temperature and also our presumption that the concentration that water being consistent is a good one.

For pure water, the values of <H3O+> and <OH-> are equal and also thus your values space both 1.0 x 10-7. If over there is an equal amount that acid and also base present in any kind of solution, the equipment is called "neutral" and the pH the the equipment is 7.

If the acid concentration is higher than the base concentration, the solution is acidic and the pH will certainly be much less than 7. If the basic concentration is better than the acid concentration, the systems is simple and the pH will certainly be greater than 7.

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The pH Scale:

The pH of a solution is identified as the negative log10

(NOTE: H+ and also H3O+ room interchangeable means to define the presence of proton in solution)

The pOH that a equipment is identified as the negative log10


= 10-pH and = 10-pOH

The pH that a neutral systems is as such 7. (-log<1 x 10-7> = 7)

The amount of the pH and pOH must constantly equal 14. This is due to the fact that the –log KW = -log(1 x 10-14) = 14.